British Prime Minister Theresa May will trigger on March 29 the two years of complex negotiations over the UK's exit from the European Union, it was announced today, nine months after the country voted to leave the bloc.

The UK's ambassador to the EU, Sir Tim Barrow, formally informed EU Council president Donald Tusk this morning to expect May's letter next Wednesday.

"We said it would be by the end of March and thought it would behelpful to say when it will happen. We want negotiations to start promptly. We expect it will be a two-year process and we are confident that is what we will achieve. So Britain will exit the EUon 29 March 2019," a Downing Street spokesperson said.

The UK expects to receive a response to Barrow's notification from the EU Council within 48 hours.

Meanwhile, an EU spokesperson told reporters in Brussels that they "ready and waiting" for the letter.

The move comes nine months after Britain voted 51.9 per cent to 48.1 per cent in favour of Brexit in a referendum on June 23, 2016.

Talks on the terms of the UK's departure and future relations with the EU are not possible until the UK formally notifies the EU it is leaving.

May has already announced that she will make a statement to the House of Commons shortly after invoking Article 50 of the Lisbon Treaty.

After May's letter reaches Tusk, he is expected to distribute draft guidelines for the negotiations to the 27 other EU member states.

He will also summon the leaders of the countries for a summit to endorse the final guidelines, expected in early May.

"Within 48 hours of the UK triggering Article 50, I will present the draft Brexit guidelines to the EU27 Member States," Tusk tweeted.

The process will give a negotiating mandate to the EU's executive arm, the European Commission, with chief EU negotiator Michel Barnier going ahead with the process of scheduling talks with his British counterpart, Brexit minister David Davis.

"The government is clear in its aims: a deal that works for every nation and region of the UK and indeed for all of Europe - a new, positive partnership between the UK and our friends and allies in the European Union," Davis, secretary of state for exiting the European Union, said.

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